1. Field of the Invention
The invention relates to a mobile radio receiver for a radio transmission system, such receiver comprising a recognition arrangement for searching a received digital signal for pulses of a frequency correction burst contained with a predeterminable frequency in the digital signal and for determining their pulse positions, and a frequency estimation arrangement for deriving frequency estimates of the predeterminable frequency.
The invention further relates to a radio transmission system includes such a mobile radio receiver.
2. Description of the Related Art
Mobile radio receivers of this type are necessary, for example, for the Pan-European mobile radio system referenced D-network in German. In this mobile radio system, for example, speech signals are transmitted in digitized form along with other digital signals by way of a Time-Division Multiple Access method (TDMA). For this purpose, the available frequency band is subdivided into separate frequency bands and a specific number of subscriber channels are accommodated in each separate frequency band in the time-division multiplex mode. The method of accessing within a radio cell is synchronous i.e. the separate time slots are organized in frames. When a connection is set up between a base station and a mobile radio station, there is the resulting problem of initial synchronization.
In the article entitled "Anfangssynchronisation der Mobilstation im D-Netz", G. Frank, W. Koch, PKI Technische Mitteilungen 1/1990, pp. 43 to 49, a mobile radio receiver of the type mentioned in the opening paragraph is described. For initial synchronization in that arrangement a frequency correction burst (FCB) is transmitted along with other information signals between a base station and the mobile radio receiver over a broadcast control channel (BCCh). This frequency correction burst is used for fine frequency synchronization of the mobile radio receiver with the base station and for coarse synchronization with regard to the time slot position. The frequency correction burst consists of a sequence of 148 zeroes which are transformed by means of a specific modulation method, for example, GMSK modulation, into a pure sine-shaped signal shifted relative to the carrier frequency of the received digital signal by the frequency of the frequency correction burst. For detecting the frequency correction burst and thus identifying what is commonly referred to as the Broadcast Control Channel (BCCh), a recognition arrangement designated as BCCh recognition is provided. The recognition arrangement searches a signal that can be derived from the received digital signal for pulses corresponding to the frequency correction burst. This search is made in a plurality of successive observation intervals. Pulses whose position in time and whose maximum value are determined are derived from the frequency correction burst. A criterion for the actually received frequency correction burst is the average height of the detected pulses. If no frequency correction burst is found, a control unit uses a new frequency channel for a further search. If the frequency correction burst has been positively recognised, the time shift of the frequency correction burst is determined on the basis of the position in time of the maximums of the detected pulses. Subsequently, a frequency estimation arrangement is activated which derives frequency estimates of the frequency of the frequency correction burst. Then, a frequency-tracking value is determined for a local oscillator arranged in the HF section of the mobile radio receiver on the basis of a rather large number of frequency estimates of the frequency estimation arrangement.